Characterization of the full elastic effect of the near surface on seismic waves

Abstract

 A method of seismic exploration uses recordings from both surface receivers and buried sensors to determine the full elastic effect of the near surface layer on an applied seismic wave. The surface receivers are arranged relative to the seismic source location so that rotational effects may be detect. Buried sensors are located so that vertical effects may be detected. This full elastic effect may be used in subsequent seismic data acquisition to reconcile the effect of the near surface so that the response of the underlying rock formations may be known.

Description

TECHNICAL FIELD OF THE INVENTION

[0001] This invention relates generally to seismic surveying, and more particularly, to determining the full elastic effect, on seismic waves, of a weathered layer near the surface.

BACKGROUND OF THE INVENTION

[0002] Seismic surveying uses an artificially induced seismic wave to infer underground geological formations. A seismic wave source approximates a delta function by striking the surface or setting off an explosion. Receivers detect the local pattern of ground motion over a short interval of time following the triggering of the seismic source. An amplified output of each receiver is recorded as a seismic trace, which represents the combined response of the layered subsurface and the recording system to the seismic source.

[0003] The response of solid rock formations in the earth to various types of applied seismic body waves is well known. A recent development in seismic surveying has been the use of three-component geophones to collect information from each type of body wave. These three-component geophones obtain data that permits particle motion to be represented as a three-component vector, with the motion being in response to compressional waves, vertical shear waves, and horizontal shear waves.

[0004] Regardless of the type of waves being detected, as in any transmission system, the signals generated by the seismic source undergo filtering by the earth during transmission to the receiver. The effect of such filtering is considered to be "noise", adverse to the desired seismic data. Thus, interpretation of seismic traces requires that filtering effects be reconciled.

[0005] One significant filtering effect that greatly affects the integrity of seismic recordings is the effect of the near surface layer. This near surface layer has properties that are very different from those of the consolidated rocks farther below. For example, the soil and the near surface rocks are affected by the elements, i.e., rain, frost, ice, temperature, and wind, which have long term as well as short term effects. Because of these different properties, the effect of the near surface layer on seismic waves is different from that of the underlying rock, and, unlike the response of seismic waves in solid rock, the response in the near surface is not well known.

[0006] Another characteristic of the near surface is that its effect is greater on shear waves than on compressional waves. Furthermore, this near surface effect is relatively overwhelming compared to the relatively subtle effects of the rock formations, with the latter being of interest to seismologists. The result is that the increasing use of multicomponent receivers has led to efforts to eliminate or reconcile the effect of the near surface. One method teaches burying the receivers below the near surface layer to avoid the near surface filtering. Another method teaches using both buried sensors and surface receivers. The buried sensor is under the source and measures the near surface effect so that a deconvolution operator is derived, which is then used to eliminate the near surface effect from the signal received at the surface. A problem with both methods, however, is that exploration results have shown that they do not consistently account for the actual behavior of seismic waves in the near surface Thus, a need exists for an improved means for determining the effect of the near surface layer on seismic waves.

SUMMARY OF THE INVENTION

[0007] An underlying principle of the invention is that the near surface layer has an elastic effect on seismic waves that is not completely determined by measuring techniques used for rock formations. For example, the near surface layer may cause the seismic source wave to move azimuthally. Thus, the basic concept of the invention is to determine the full elastic effect of the near surface layer on the seismic source. This permits a complete deconvolution operator to be obtained.

[0008] Another aspect of the invention is a method for determining the effect of the near surface layer on an applied seismic wave. The full elastic effect on the applied signal, including the effect of compressional waves, shear waves, and surface waves, is measured. This measurement can be used in during seismic data acquisition to reconcile for the effects of the near surface layer. Surface receivers, as well as buried sensors, are used to detect the seismic waves at selected points along its path down through the near surface, through the underlying rock formation, and back up through the near surface. The surface receivers are placed so that azimuthal deviations of the seismic source wave are detected.

[0009] Another aspect of the invention is a method for obtaining a deconvolution operator, which represents the filtering effect of the near surface layer. This deconvolution operator can be used to eliminate the near surface effect on the seismic trace so that underlying rock formations can be more successfully known. The deconvolution operator is derived from measurements of the seismic source using surface receivers and buried sensors that are arranged to detect both rotational and vertical filtering effects.

[0010] Another aspect of the invention is a system of signal sources and receivers for detecting the full elastic effect of the near surface on an applied seismic wave. Sources on the surface, buried sensors, and surface receivers are placed in a three dimensional pattern. The surface receivers are placed so that they receive source signals that deviate from a projected direction. The buried sensors are placed so that they receive downgoing source signals after filtering by the near surface and upcoming source signals after the first filtering by the near surface and after passing through the rock formation under the near surface.

[0011] A technical advantage of the invention is that tests to determine the effects of the near surface layer are greatly improved. Elastic effects that were previously undetected are now detected, and a complete deconvolution operator is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as modes of use, and further advantages, will best be understood by reference to the following Detailed Description of illustrative embodiments when read in conjunction with the accompanying Drawings.

FIGURE 1 is a plan view of seismic source locations and receivers arranged in accordance with the present invention.

FIGURE 2 is a sectional view of a single seismic source and a single surface receiver, with two buried sensors, illustrating the path a seismic wave might follow from the source to the surface receiver.

FIGURE 3a illustrates a tube insertion means for locating the buried sensors of FIGURES 1 and 2.

FIGURE 3b illustrates a strain gauge associated with the tube insertion means of FIGURE 3a.

FIGURE 4 is a flow diagram of the method of the present invention.

FIGURES 5a and 5b illustrate how a source signature is determined for purposes of obtaining a deconvolution operator in accordance with the present invention.

FIGURE 6 is a flow diagram of an alternate method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0013] In general, the invention may be implemented to any time prior to, or simultaneously with, seismic data acquisition for actual exploration. The preferred method, however, is to implement the invention at about the same time as the acquisition of the seismic data because the invention's measurand, i.e., the near surface effect, is a function of present conditions as well as stable properties. Changing conditions such as moisture and temperature cause the near surface effect to change from one time to another.

[0014] As discussed below in connection with FIGURE 1, seismic surface receivers are arrayed on points corresponding to one-half of the receiver array intervals anticipated for subsequent exploration data acquisition. Connections are identical with later data acquisition. Sources are placed on source locations corresponding to double the receiver intervals and operated at three times the field effort anticipated for exploration. Buried sensors are placed along at least one shot line and along one side of the surface receiver array.

[0015] FIGURE 1 is a plan view of a preferred system used in accordance with the present invention on a test field on the earth's surface. There are three basic components of the system: seismic sources on the surface, buried sensors, and surface receivers. The sources, sensors, and receivers comprise a three dimensional spread, with x and y on the surface and z down into the ground. For a complete study of a particular exploration field, it is anticipated that the system of FIGURE 1 will be deployed at all extremes of the field and at selected intermediate locations.

[0016] In the preferred embodiment, each seismic source is a full waveform source that generates shear and compressional waves. The full waveform source generates a wave that is "directed". If there were no near surface effect, the wave could be expected to follow a projected path along a straight line in a known direction, i.e., the shot line. An example of such a source is the ARIS source developed by ARCO Oil & Gas Company.

[0017] In one embodiment of the invention, both the surface receivers and the buried sensors are three-component particle motion receivers, such as are well known in the art of seismology. These receivers detect compressional waves, vertical shear waves, and horizontal shear waves. Assuming that the receivers are three-component receivers, misalignment from the shot line is overcome using the method set out U.S. Patent No. 4,803,669. In another embodiment of the invention, the buried sensors are strain gauges, as discussed below in connection with FIGURES 3a and 3b.

[0018] The locations of the surface seismic receivers form a surface receiver array 12. Typically, the length and width of surface receiver array 12 are in the order of one mile. The primary consideration is that the length of either side of surface receiver array 12 be sufficient to determine azimuthal deviation of seismic waves, as explained below.

[0019] Individual surface receivers 13 are located at evenly spaced intervals within surface receiver array 12 to form a matrix. In FIGURE 1, the receivers are identified in matrix notation as 13_i,j, where i = 1 - 8 and j = 1 - 8. In theory, the number of receivers per line is a function of accuracy of the measurements obtained, but at least two must be used per line. In practice, the number of receivers used is also determined by the desired length of either side of surface receiver array 12 and by the size of receiver groups such as are commonly used in the industry. For example, a line of receivers one mile long might use group intervals of 110′ to 220′, with each group having eight receivers, resulting in 48 to 24 receivers.

[0020] The configuration of FIGURE 1 of surface receiver array 15 is the preferred embodiment, although a simpler verson of surface receiver array 12, such as an L-shaped array along lines a-b and a-c, could accomplish the same result. However, the arrangement of rows and columns of FIGURE 1 is preferred in order to obtain increased energy of the signal reflected to the surface. In implementing the invention it is possible to set up the complete surface receiver array 12 at one time, or alternatively, to set up a limited portion appropriate for the particular seismic source. A minimum surface receiver array 12 during use of the invention consists of a line of surface receivers, such as either line a-b or a-c of FIGURE 1.

[0021] The locations of the seismic sources form a source pattern generally designated as 14. Individual source locations 15 of seismic sources form at least two orthogonal shot lines, a-d and a-e. Source pattern 14 is located off the right angle formed by receiver lines a-b and a-c. One side of the source pattern extends off line a-b and the other side extends off line a-c. In other words, source pattern 14 is in the opposite quadrant from surface receiver array 12. Source locations 15 begin off end of a receiver line in receiver array 12 and are walked up to the beginning of each receiver line. A typical distance of the farthest source location, i.e., the location at d in FIGURE 1, is in the order of 2 miles from surface receiver array 12.

[0022] An enhancement of the invention includes additional shot lines, lines a-f, a-g, and a-h, along angles between lines a-d and a-e. These additional shot lines permit a more complete indication of the near surface effect. Although seismic injections along shot lines a-d and a-e will detect near surface elastic effects as straight line deviations, the use of the additional shot lines will detect more complicated deviations, such as curves.

[0023] The relative locations of surface receiver array 12 and source pattern 14 is consistent with the underlying concept of the invention, that the near surface layer has elastic effects on the source signal that cause the source wave to deviate from its projected path. Surface receiver array 12 permits a seismic source wave to be injected so that at least one surface receiver 13 is in-line with the source wave and at least one surface receiver 13 is off-line. By "in-line" with the source wave is meant that the surface receiver 13 lies along the projected direction of the source wave. All other receivers not on that line or an extension of that line are considered "off-line" with respect to that source wave.

[0024] Buried sensors 19 are placed in a line parallel to one of the lines of source pattern 14 and in two lines parallel to the sides of surface receiver array 12. The determination of whether to bury all sensors 19 at once or as needed is a practical consideration. Preferably, for each seismic injection at a source location 15, at least one buried sensor 19 is near the source location 15 and at least one buried sensor 19 is near the line of surface receivers 13 orthogonal to the direction of the seismic injection.

[0025] FIGURE 2 is a diagrammatic vertical section through the earth illustrating the invention, but showing only one source location 15_5,1, one surface receiver 13_1,5, and two buried sensors 19₁ and 19₄. A seismic source 21 at location 15, the surface receiver 13_1,5, and data recording and processing station 23 are all positioned on an earth surface, S. Appropriate electrical communication is established between these devices, as is well known in the art of seismic surveying.

[0026] For purposes of example, in connection with FIGURE 2, it is assumed that sensors 19 are three-component seismic receivers, but implementation of the invention using some other type of sensors, such as a strain gauge, is not substantially different. A strain gauge is a type of transducer that relies on an applied strain to change its resistance. A strain gauge operates in accordance with the principle that resistance of a conductor is proportional to its resistivity, cross sectional area, and length. Thus, if one or ore of these characteristics change, resistance changes. Strain gauges are available that use metal transduction elements, and the application of strain simply changes their length and their cross sectional area to alter the resistance value. Certain substances, however, exhibit a piezoresistive effect, so that application of strain affects their resistivity. Any of a number of types of strain gauges may be used, with the important characteristics being that the strain gauge be capable of measuring strain in all directions. This permits the complete elastic effect of the near surface to be measured.

[0027] One sensor 19₁ is buried in the earth near source 21 to a depth at which the signature of the source 21 is substantially modified by the filtering effect of the near surface, NS, under surface S. The accuracy of the invention is a function of how well the near surface effects, as opposed to rock formation effects, are measured, thus the buried sensors 19 should be buried at the depth that most closely represents that line of demarcation. The "substantially modified" depth may be satisfied by locating sensor 19₁ at or below the base, B, of the NS as shown, where B represents the level of bedrock. This depth may also be satisfied with shallower placements. For example, if the depth of the base, B, is undetermined, buried sensors could be placed beneath the several upper most layers of soil. Also, using an uphole survey, the depth where there is the greatest effect of the near surface on wave velocity can be determined, and this depth used to approximate the depth of the near surface layer.

[0028] If it is assumed that the near surface NS constitutes approximately the same filter from source 21 to sensor 19₁ as from sensor 19₄ to surface receiver 13_1,5, it is possible to practice the invention with a single buried sensor. In this connection, the effects of the near surface are less drastic from sensor 19₄ because the energy intensity is lower and frequencies are lower. Thus, it may be convenient to use only sensor 19₁, and assume that the other sensor would have the same response. Nevertheless, the purpose of the invention being the determination of the full elastic effect of the near surface, and the assumption being that these effects are anisotropic, the use of receivers under both the source and the surface receivers is preferred. Thus, a second buried sensor 19₄ is placed near the surface receivers that will receive whatever seismic wave is being injected.

[0029] The buried sensors may be placed beneath or in the near surface, as shown in FIGURE 2, by a number of means. One means is the use of drilled boreholes. If a three-component receiver is used, it must be fixed in the borehole so that it will move in response to the surrounding medium, and must be oriented relative to the azimuth with respect to the source. If a strain gauge is used, it must be fixed in the borehole so that it will deform with the medium.

[0030] FIGURE 3a shows another means for placing buried sensors 19₁ and 19₄, as well as all other receivers 19, in the ground. As shown in FIGURE 3a, each sensor 19 may be inserted into the ground by means of an insertion tube 31. Electrical leads 34 to sensor 19 are brought to the surface through the inside of insertion tube 31 and attached to the seismic recording system. Insertion tube 31 and its associated sensor 19 may also be left in place for use during seismic data acquisition. After preproduction testing and seismic data acquisition have been completed, each insertion tube 31 and sensor 19 may be retrieved for reuse. Specific examples of insertion tubes 31 are threaded jointed tube sections or coiled tubing. An advantage of using insertion tubes 31 is that no prior drilling is required.

[0031] A convenient implementation of a strain gauge used in accordance with the present invention shown in FIGURE 3b, which shows a single strain gauge element 33. As is well known in using strain gauges as sensors, various configurations of these elements in different planes may be used. A complete strain gauge is fitted with patterns of strain gauge elements 33 in a configuration designed to measure the complete stress in the surrounding medium. Strain gauge element 33 is used as a sensor 19 and comprises a hollow cylinder at one end of insertion tube 31. Strain gauge element 33 is closed and protected by an attached strain isolated point at its lower end. Strain gauge element 33 is inserted into the ground by means of insertion tube 31 at its upper end, which strain isolated. The electrical leads 34 for each strain gauge element 33 are brought to the surface through insertion tube 31.

[0032] Referring again to FIGURE 2, the path of an elastic wave, W, generated by source 21, as received by a surface receiver 13 is illustrated. The wave proceeds downward through the near surface NS to buried sensor 19₁, then through the rock formation, F, to point A, where it is reflected upward. The wave then proceeds up through formation F and through the near surface NS to the buried sensor 19₄ to the surface receiver 13. The downgoing and upgoing paths of the wave are refracted at the base B of the near surface NS because of the difference in velocities between the near surface NS and formation F.

[0033] For each received signal at receiver 13 that follows a path such as illustrated in FIGURE 2, the effects of the near surface NS may be distinguished by using the methods of the invention, described in further detail below.

[0034] The method of the invention is best understood with reference to both FIGURES 1 and 2, as well as to FIGURE 4, which is a block diagram of the steps performed in accordance with one embodiment of the invention. In general, regardless of whether the ultimate goal of the method is deconvolution or some other means for reconciling near surface effects, combined measurements from the surface receivers and the buried sensors in accordance with the invention can be used in a data processing system to determine a number of near surface characteristics. These include reflections of all waveforms, offsets of converted waves along with polarity changes, azimuthal variations in energy level and arrival time of different waveform components, relative energy levels of surface and buried sensors along with rotational effects, indications of near surface waveguide effects due to elevation changes, and variations in elastic properties. These characteristics can be compiled for a comprehensive determination of the full elastic effect of the near surface.

[0035] In accordance with this general objective, Step 41 is generating a seismic wave, i.e., a seismic injection, on the earth's surface. The source locations 15 are individually and sequentially excited by source 21 so that only one seismic wave is injected into the earth at any one time. Thus, source 21 can be moved from one position to another for a sequence of seismic recordings.

[0036] Generation of the seismic source wave is accomplished with a three-component seismic source, such that the shear waves have a projected direction along a path. It is this projected path that may deviate, and thus indicate rotational effects of the near surface.

[0037] Steps 42 and 43 are receiving the seismic wave with surface receivers 13 and buried sensors 19. All three-component receivers are compensated to reconcile for misalignment with respect to the seismic wave direction, using the method of U.S. Patent No. 4,803,669. Thus, any remaining deviation can be assumed to be due to the effects of the near surface.

[0038] For the first seismic injection, surface receivers 13 are set up in accordance with the above described embodiments or an equivalent. Typically, at least one receiver line orthogonal to the direction of the seismic wave is used. A first buried sensor 19 is placed near the source location and a second buried sensor 19 placed near the line of surface receivers 13 that is orthogonal to the source location. For subsequence seismic injections, the buried sensor 19 near the source location may be omitted. The direction of the seismic injection determines which line of surface receivers of array 12 will be used, and also, which buried sensor 19 associated with the surface receivers is used.

[0039] When a seismic wave is injected into the ground, any tendency of the near surface NS to cause the wave to deviate from a straight line will be detected by source receiver spread 12. Specifically, for a source at location 15_5,1 if the near surface NS has no rotational effects the strongest response, i.e., the response having the greatest amplitude, would be at the receiver closest to the source. In FIGURE 1, this would be the receiver 13_1,1 at point a. On the other hand, certain elastic effects of the near surface NS may cause the seismic wave to rotate azimuthally with respect to the source. For example, the strongest signal might be at receiver 13_1,5. This indicates a rotational effect of theta degrees from the unrotated shot line.

[0040] So far, a clockwise rotational effect of the near surface NS has been assume. To detect counterclockwise, as well as clockwise rotation, a second seismic wave may be injected at a second location, such as at 15_1,5. This second location is preferably on a shot line a-e that is orthogonal to the shot line a-d formed by the source at 15_5,1 and surface receiver 13_1,1. The receiver lines to be used are orthogonal to those used to detect the clockwise effects. Thus, for counterclockwise effects, the shot line has been rotated ninety degrees, and the receiver line has been rotated ninety degrees in the opposite direction. A second seismic wave is injected at location 15_1,5. If the strongest response is at receiver 13_3,1, the indication is a rotational effect of angle alpha.

[0041] In accordance with the above procedure, surface receiver array 12 may be set upt completely as shown in FIGURE 1. Alternatively, fewer surface receivers 13 than required for the complete array may be used. As indicated above, for example, receiver array 12 may be comprised of an L-shaped pattern along lines a-b and a-c. If this is the case, it is possible to set up first one line of the L-shape for receiving seismic shots from an orthogonal direction, and then the other line of the L-shape for receiving orthogonal seismic shots Thus, for example, a line of receivers 13 could be set up along line a-b with seismic source locations 15 along line a-d, then the process repeated with receivers 13 along line a-c with source locations 15 along line a-e.

[0042] Regardless of whether clockwise or counterclockwise effects are being sought, responses are obtained from buried sensors 19. For the source signal originating at location 15_5,1 for which the strongest signal is received at surface receiver 13_1,5, signals are also received at buried sensor 19₁ or at 19₄, or at both.

[0043] Step 44 is interpreting the received signals, which involves selecting the strongest received signal from surface receiver array 12. The location of the receiver 13 associated with this signal is then used to determine the rotational effect of the near surface NS.

[0044] Step 45 is obtaining the effect of the near surface NS as a deconvolution operator. In general terms, deconvolution is a method of recovering an input signal from an output signal, using a deconvolution operator for each source wave. Deconvolution may be expressed mathematically in terms of the various signals and filters. Thus,
R_s = S_s * L₁ * E * L₂,
where R_s is the uncompensated seismic record received, as in step 43, at the earth's surface; S_s is the primary source signature on the surface, L₁ is the filtering effect of the near surface NS beneath source 21; E is the response of the underlying earth formation F; and L₂ is the filtering effect of the near surface NS beneath surface receiver 13.

[0045] If S_w = S_s * L₁, which is the signature at the buried sensor under the source, the object of deconvolution is to find an inverse filter, i.e., a deconvolution operator, S′_w, which is convolved with R_s as follows:
R_s * S′_w = E * L₂.
A second deconvolution operation, L′₂ may be found to represent an inverse filter for L₂, and thereby yield only E. It is well known in mathematics that a deconvolution operator convolved with its originating signal results in an impulse function. Thus, if the originating signal is known, the deconvolution operation may be obtained.

[0046] To obtain S_w, an arrangement similar to that shown is FIGURE 5a is used. It is well known in the art of seismology, in connection with obtaining a vertical seismic profile, how to discriminate between downgoing seismic energy arriving at a given depth in a borehole from upcoming energy reaching the same depth from nearby reflectors. This same technique is used in the present invention. A vertical line of individual sensors 51, 52, and 53 collectively form sensor 19. Downgoing signals, DS, reach sensors 51, 52, and 53 directly from source 21. Upcoming signals, US, reach sensors 51, 52, and 53 after having been reflected at F.

[0047] FIGURE 5b illustrates a plot of the downgoing and upcoming signals of FIGURE 5a. The plot is on a time scale, with a trace for each receiver. Because of moveout, the successive arrival times of the direct signals form a line having a negative slope. The arrival times of the reflected signals form a line of positive slope. The intersection of the two slopes makes it possible to distinguish downgoing from upcoming signals.

[0048] Using the receivers of FIGURE 5a and the method illustrated in FIGURE 5b, a source signature S_w can be identified. Any of buried sensor 19 described above may be comprised of several receivers, as shown in FIGURE 5a. In other words, the above references to a buried sensor 19 should be read to include a vertical receiver group, such as receivers 51, 52, and 53.

[0049] In one variation of the deconvolution method of the invention, it is assumed that the source signal from source 15_5,1 as detected by receiver 13_1,5 has undergone filtering by the near surface NS. Assuming a generally linear elastic behavior of the formation F, the source signal is relatively unchanged between receiver locations 19₁ and 19₄. In other words, the downgoing signals at sensor 19₁ should closely approximate the upgoing signals at sensor 19₄. Thus, the deconvolution operator for the upcoming signal L₂ is considered to be the same as the deconvolution operator for the downgoing signal L₁. This permits the use of only one buried sensor. In the preferred embodiment, however, both buried sensors are used to obtain a more complete indication of the full elastic effect of the near surface.

[0050] The deconvolution operator obtained from the buried sensors is used to indicate the near surface effect in the vertical plane. When this deconvolution operator is combined with the effects indicated by the surface receivers 13, the full elastic effect of the near surface is indicated. This elastic effect can then be used in subsequent seismic data acquisition to isolate E, the response due to rock formations.

[0051] If a strain gauge is used rather than a three-component receiver, the method of the invention is basically the same. The advantage of using a strain gauge is that a more complete elastic effect is detected, and hence the deconvolution operator more truly represents the near surface effects. Differences in receiver-amplifier characteristics of the equipment can be compensated by using techniques well known int he arts of seismology and electronics.

[0052] To measure the elastic effects of a particular field, the steps illustrated in FIGURE 1 are used to generate a seismic source wave at different source locations, in accordance with the pattern of FIGURE 1 and the variations discussed above. For each source location, the surface receivers and buried sensors used to detect the seismic wave are varied in accordance with the above discussed scheme of detecting azimuthal as well as vertical near surface effects.

[0053] FIGURE 6 illustrates an alternative method of the invention. The steps of this method are the same as discussed in connection with FIGURE 4, but rather than deconvolving the signals, after a tendency of the near surface to azimuthally deviate seismic waves is determined with the source receiver spread 15, this tendency is used to place the production source and receiver spread. In other words, once the azimuthal direction for the effect of the near surface NS is known, data acquisition tests can be oriented in that direction, causing near surface effects to simply drop out. It is then assumed that distortions are from rock formations and not from the near surface.

[0054] Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternative embodiments, of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It is, therefore, contemplated that the appended claims will cover such modifications that fall within the true scope of the invention.

Claims

1. A method for determining the full elastic effect of the near surface layer on a seismic source wave, comprising the steps of:
placing a plurality of surface seismic receivers on the surface of the earth;
placing at least one buried sensor beneath the surface of the earth at a depth where the filtering effect of the near surface is substantial;
injecting a seismic wave into the ground at a current source location remote from said surface receivers, such that at least one surface receiver is in-line and at least one surface receiver is off-line with respect to the shot line of said seismic wave;
receiving said seismic wave with said surface receivers and said buried sensor;
generating from said buried sensors and said surface receivers electrical representations of said received seismic waves; and
interpreting said electrical representations to determine the effect of the near surface layer on said seismic wave.

2. The method of Claim 1, wherein said step of placing surface receivers comprises placing a line of surface receivers, and wherein said step of injecting a seismic wave comprises directing said wave in a direction orthogonal to said line of surface receivers.

3. The method of Claim 1, wherein said step of placing surface receivers further comprises placing multiple lines of said surface receivers.

4. The method of Claim 1, wherein said step of placing buried sensors comprises placing one receiver beneath said seismic source location and one sensor beneath said surface receivers.

5. The method of Claim 1, and further comprising repeating said steps, wherein said current source location is rotated ninety degrees in a first direction and said surface receivers are arranged in a line rotated ninety degrees in a second direction, and wherein a buried sensor near said surface receivers is used.

6. The method of Claim 5, and further comprising repeating said steps wherein said current source location is moved to a location between said previous source locations, and said surface receivers are arranged in a line orthogonal to the direction of the seismic shot line from said current source location, and a buried sensor near said surface receivers is used.

7. The method of Claim 1, and further comprising the step of aligning said surface receivers to compensate for misalignment with respect to said shot line.

8. The method of Claim 1, wherein said step of placing a plurality of buried sensors includes using an insertion tube.

9. The method of Claim 1, wherein rather than a plurality of buried sensors, a single sensor for said current source location is used, said sensor being located near said current source location.

10. The method of Claim 1, wherein said interpreting step includes detecting the strongest signal from said surface receivers to determine any rotational effect of the near surface.

11. The method of Claim 1, and further comprising the step of obtaining a deconvolution operator in response to said interpreting step.

12. The method of Claim 1, and further comprising the step of locating seismic data acquisition equipment in response to said interpreting step.

13. A method for obtaining a deconvolution operator, which represents the effect of the near surface on a seismic source wave, comprising the steps of:
generating a downgoing seismic wave at a source location on the surface of the earth, such that said wave is directed in a particular projected wave direction;
receiving said wave beneath the surface of the eath at a depth where the wave is substantially modified by the filtering effect of the near surface layer, using a buried sensor;
receiving said wave at the surface of the earth at a plurality of locations, using surface receivers, wherein at least one of said receivers is in-line with said projected wave direction and at least one of said receivers is off-line with respect to said projected wave direction; and
isolating the effect of said near surface, using the signals received from said buried sensor and said surface receivers to obtain said deconvolution operator.

14. The method of Claim 13, wherein said step of receiving said wave beneath the surface of the earth comprises using a vertical line of receivers for discriminating between downgoing seismic waves and upcoming reflections.

15. The method of Claim 13, wherein said step of receiving said wave beneath the surface of the earth comprises using a buried sensor located near said source location.

16. The method of Claim 13, wherein said step of receiving said wave beneath the surface of the earth comprises using a buried sensor located near said surface receivers.

17. The method of Claim 13, wherein said step of receiving said wave beneath the surface of the earth comprises using a first buried sensor near said source location and a second buried sensor near said surface receivers.

18. The method of Claim 17, wherein said step of isolating the effect of the near surface comprises isolating the effect of the near surface on downgoing waves from the source and the effect of upcoming waves from a reflector beneath the near surface.

19. The method of Claim 13, and further comprising the step of repeating said wave generating step and said receiving step for various source locations, surface receiver locations, and buried receiver locations until a desired accuracy of the near surface effects can be expected.

20. The method of Claim 13, and further comprising the step of changing said location of said source to a new source location and of using surface receivers orthogonal to the projected wave direction from said new source location.

21. The method of Claim 13, and further comprising the step of inserting said buried sensors using an insertion tube.

22. The method of Claim 21, wherein said a portion of said insertion tube is used as a strain gauge.

23. A seismic wave generating and detecting system for use in determining the elastic influence of the near surface on seismic waves, comprising:
a plurality of surface seismic receivers, said surface receivers forming at least one line of receivers;
a seismic wave generator for generating seismic waves, said seismic wave generator being placed in a location remote from said surface seismic receivers;
at least one buried sensor, said buried sensor being buried at a depth where the effects of the near surface are substantial;
a date processing station for receiving and interpreting said seismic waves from said surface receivers and said buried sensors;
communication lines for transmitted said signals between said surface receivers and said buried sensors and said data processing station.

24. The apparatus of Claim 23, wherein said surface receivers form a two dimensional pattern.

25. The apparatus of Claim 24, wherein said surface receivers form an L-shaped pattern.

26. The apparatus of Claim 24, wherein said surface receivers form a two dimensional array having a number of columns and rows.

27. The system of Claim 23, wherein said buried sensors are three-component seismic receivers

28. The system of Claim 23, wherein said buried sensors at strain gauges.

29. The system of Claim 23, and further comprising an insertion tube for inserting said sensors.

30. The system of Claim 29, wherein a part of said insertion tube is used as a strain gauge.

31. The system of Claim 24, wherein said buried sensor is buried near said surface seismic receivers.

32. The system of Claim 24, wherein said buried sensor is buried near said seismic wave generator.

33. The system of Claim 24, wherein a first buried sensor is buried near said surface seismic receivers and a second buried sensor is buried near said seismic wave generator.

Drawing